1. Field of the invention
The present invention relates to DC lighting systems, and more particularly, to a device for periodically alternating the polarity of a DC power signal used to drive a DC light bulb.
2. Description of the Background
Fluorescent lighting systems are in wide use due to the high-efficiency cool light which is generated. Conventional fluorescent lighting systems may be one of two types including those operated from AC power, or those operated from DC power. AC-powered systems are the most common in houses, factories and public places. However, AC-powered systems are prone to flicker, which may result in harm to the eyes. In contrast, the continuous DC signal applied to DC systems solves the problem of flicker because the bulb is always biased with a signal of the same polarity. Unfortunately, the advantage of uniform polarity does not come without cost. When a constant power signal is applied to a fluorescent bulb, severe aging and eventual degradation of the cathode will result.
It would be greatly advantageous to provide a solution to the above-described problem, and co-pending U.S. Patent Application Ser. No. 07/575,425 attempts such a solution with a device for periodically alternating bulb polarities of a DC fluorescent lighting system. Disclosed therein is a means for randomly setting the bulb polarity of a fluorescent light bulb prior to application of power. When power is applied, either bulb terminal has an equal chance of becoming the cathode. This results in uniform degradation of the cathode over time.
It would be equally advantageous if the bulb polarity could be alternated more frequently than at each power-up. However, for this a solution must be provided for the problem of flicker which is inherent in AC fluorescent lighting systems.